Glancing over RFI sources

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The Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) instrument onboard SMOS is a Y-shaped antenna with a total of 72 receivers distributed along its three arms and central body. Each receiver captures the thermal radiation in the microwave L-band, more specifically in the protected passive band comprised between 1400 and 1427 MHz. Since the emission within this band is prohibited by the International Telecommunications Union (ITU), no relevant external interferences were expected before SMOS launch (2009). Nevertheless, the real situation is that the Radio Frequency Interferences (RFI) are present in large areas of Europe and Asia leading to low quality measurements. Moreover, due to the MIRAS interferometric processing, RFI sources located far away, even beyond the MIRAS Field of View (FOV), can contaminate large portions of the MIRAS image.

Retrieved L2 values. Ascending passes. Year 2012

Figure 1. Map of the number of retrieved L2 SSS values during 2012 for ascending passes. White areas have no valid L2 SSS values along 2012.

For a given zone, RFI signals can be classified in terms of the mean life time of the interference as transient emissions or permanent emissions. The former have a limited temporal influence and are mainly produced by mobile sources (for instance ships in open ocean). The latter have a strong effect and may even systematically prevent  the retrieval of salinity or soil moisture.

Our Web Map Server service (based on ncWMS and Godiva2 developed by Reading e-Science Centre at the University of Reading) can be used to reveal the spatial distribution of persistent RFI over ocean. The presence of a RFI source reduces the number of valid measures in the zone. Thus, affected zones can be detected by mapping the L2 used measures parameter.

Retrieved L2 values. Descending passes. Year 2012

Figure 2. Same as figure 1 for descending passes.

The comparison of figures 1 and 2 (images can be zoomed by clicking on them) shows that the affected areas are slightly different, depending on the orbit pass. Nevertheless, seas around Europe, Madagascar, Bay of Bengal, Arabian Sea, East and South China Seas and Sea of Japan are systematically RFI contaminated in both ascending and descending passes.

L-band transmitters in islands (as the Solomon, Ascension, or Barbados) may also prevent accurate retrievals in regions of great interest as the Amazon Plume.

The following two videos show the monthly evolution of RFI sources for ascending and descending passes. (Similar animations can be created on-line using our above mentioned Web Map Server).

The small number of retrieved values of SSS for descending passes in the Northern Hemisphere during the October-January period is related to Sun activity. Transient RFI can also be easily detected in monthly animations: the large RFI burst observed in Madagascar between August and November, the intermittent nature of the RFI located in front of Brazilian coast (Natal), or the fading, as seen by descending orbits, of the RFI affecting the coast of Namibia. It is also possible to detect short RFI episodes such as that which took place in February at Samoa Islands.


Video 1. Monthly evolution of RFI distribution along year 2012. Ascending passes. Use the controls to display full screen animation

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Video 2. Monthly evolution of RFI distribution along year 2012. Descending passes.

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Geolocation of distant RFI sources

Usually, an approximate location of an artificial RFI source can be obtained by inspection of the brightness temperature snapshots contained in the Level 1C products plotted over a map. The source is clearly visible and it crosses the extended alias free FOV while SMOS moves over it.

Sources of RFI marked as A,B (ascending passes) and C (descending passes) are located away from the area where its effect is clearly noted.

Figure 3. Sources of RFI marked as A,B (ascending passes) and C (descending passes) are located away from the RFI source location.

Nevertheless, there exist some RFI that perturb the SMOS measures in areas far away from the source. This is the case of targeted emissions in the microwave L-band that, under some circumstances, can intercept the SMOS path up to 3000 km away from its origin. These kind of RFI sources could be mainly related to a narrowly focused radio beam coming from microwave links. In these cases the interference could be captured by SMOS near the horizon visible.

Inspection of the permanent RFI sources shown in figures 1 and 2 reveals that, at least, three of them are good candidates to be generated by sources located close to the visible horizon. The affected areas are shown in figure 3 and they are identified by letters A, B and C. All of them are open ocean areas that do not appear to contain permanent sources of RFI. The Region A is located south of Clipperton Island, the region B is located at the east of Solomon Islands, and the region C is centered at the north of Fiji Islands.

Tx Snapshot taken on December 17, 2012 at 13:00:58 UTC. Ascending orbit.

Figure 4. Tx snapshot taken over region A on December 17, 2012 at 13:00:58 UTC. Ascending orbit.

Figure 4 shows a pure X-polarized snapshot in the antenna reference frame (i.e. as seen by SMOS at 755 km altitude from the Earth’s surface). This snapshot corresponds to the area labelled as A in figure 3. The brightness temperature values are located in the so called fundamental hexagon that results from the image reconstruction process. The green area corresponds to the Earth’s surface and the upper purple line delimits the Earth-sky horizon as seen by SMOS. The thin dashed lines delimit 6 replicas (also known as aliases) of the Earth horizon around the green area. In this case, the area delimited by points marked as A’-B’-C’ shows the RFI source (red spot close to C’ point). This area is actually a replica of that which actually contains the RFI source (A-B-C).

The level 1C products only contain brightness temperatures in the extended alias-free FOV (the area inside the intersection between the grey line and the fundamental hexagon). This information is not enough to locate distant RFI sources located close to the horizon line (the outermost part of the green area). This kind of RFI sources are revealed only in the replica parts of the hexagon. Such information must be extracted from the level 1B products.

Fundamental hexagon projection over the Earth surface

Figure 5. Fundamental hexagon projection over the Earth surface. Click here to see a similar image in Google-Earth

The usual procedure is to identify the position of the RFI in antenna coordinates (ξ, η) and project it into the Earth surface. This must be done with special care when locating sources close to the horizon because a small misplacement in ξ or η values implies large distances over the Earth surface. Note though that the precise geolocation of the source is out of the scope of this post. Instead, we adopt a coarse-grained procedure: the entire hexagon together with the horizon line and the lines corresponding to ξ=0 and η=0 are projected into the earth surface to delimit the approximate location of the source. The projection result is presented in figure 5 showing that the source of the RFI that causes poor SSS retrievals in region A is located in the south of Guatemala or north of Honduras. A Google-Earth version of this figure can be downloaded to better inspect the zone.

RFI sources corresponding to regions B and C are found in a similar way. Both RFIs seem to be caused by the same source (or two close sources) located in the southeast part of the Solomon Islands.

Additional information about this topis is given in this Technical Note